Skip to main content

Full text of "Text Book Of Mechanical Engineering"

See other formats


Angle of Friction.                           561

to the surface, but if the surface be rough, the reaction is inclined
to the normal by the friction angle, in a direction away from the
/#//P, and the latter must now be increased by Fn in order to
move the body. If not on the point of sliding, the obliquity of R
may be anything less, down to zero. Two cases are shewn for the
inclined plane, P being directed up or down the plane, but its
value may always be found by force diagram. In moving up the
plane, total pull must balance gravity + Fn, but in moving down
the plane must balance Fn - gravity.

Example 57.— A road engine weighs 12 tons. Find (i) tractive
force of engine to pull 48 tons behind it on a level road, and (2) the
load drawn up a I in 10 incline. Coefficient of traction =150 Ibs.
per ton.

(1)        Tractive force =(12 + 48) 150 = 9000 Ibs.

(2)        P x length = W x height     and P = W x -^
Also R will be found to = -995 W

Tractive force to balance gravity =       —~    = 224     Ibs. per ton.
Tractive force to overcome friction = 150 x '995 = 149*25 ,,         „
Total tractive force =                     373*25 „        „

But the engine only exerts 9000 Ibs.

.*. Total load on incline including engine = -— — = 24*11 tons.
and Load drawn exclusive of engine = 24*11 — 12 = 12*11 tons.

Diminishing Friction by Lubrication.—Spongy metals
like cast iron, brass, and white metal decrease frictional resistance
considerably, but the best results are only obtained by the appli-
cation of unguents.

Lubricants may be solid, as blacklead; semi-solid, as greases
and fats; and liquid, as oils. f Body' for support, and fluidity
to avoid resistance, are both essential requisites, and a careful
choice must be made between extremes. The following are the
unguents used for various purposes:—